Materials for thermoplastic lenses are known from, for example, the references JP 06 322039-A, JP 07 053430-A, U.S. Pat. No. 5,409,975, JP 06 172508-A, JP 06 145317-A, U.S. Pat. No. 5,319,149, U.S. Pat. No. 5,306,832, JP 06 263863-A, JP 06 239781-A, JP 05 331277-A, JP 05 194719-A, JP 05 078567-A, JP 05 078467-A, WO 92/11319-A1, JP 04 214751-A and JP 02 070712-A.
Aromatic copolymer-polycarbonates are described in these references which, because of their high glass transition temperatures, their good transparency and their easy working properties, are suitable for lenses. The disadvantage of such polycarbonates lies in their insufficient UV resistance and inadequate scratch resistance and surface hardness. For these reasons lenses made of polycarbonates cannot be practically employed without special protective layers.
Such protective layers and their application to a polycarbonate substrate are also known, for example from the references EP 588 508-A1, JP 07 108560-A, DE 692 00 870-E, U.S. Pat. No. 5,008,148, JP 60 250925-A, JP 06 206231-A, DE 42 38 279-A1, JP 05 093170-A and JP 93 069845-B.
An illustrative impression regarding the latest prior art in connection with automobile headlight lenses is provided by an article published in the magazine K Plastic&Kautschuk, No. 17 of Aug. 25, 1995, page 13, Giesel, publishers, Isernhagen/Hannover (Germany). It can be seen from this that large investments in elaborate production installations are made for coating polycarbonate lenses.
An essential disadvantage of the described protective coatings or "hard coats" lies in that they make the final product more expensive and that relatively large amounts of scrap are generated during back-spraying of protective foils or during coating with hard coats. Coating is only suitable for relatively flat and simple parts so that blinkers, for example, cannot be integrated into a headlight structure for automobiles without special arrangements.
A particular danger lies in the crack formation of the protective layer during use, so that the coating can become partially separated from the substrate. This effect makes the lens look dirty and cloudy and allows the entry of chemical substances, for example road salt, between the protective layer and the substrate, which results in the dimming of the lens.
Also, scratch-resistant coatings in accordance with the prior art only cover the edges of the formed parts insufficiently. This is an additional source of the above mentioned resulting damages.